1. Field of the Invention
The present invention relates to an anisotropically electroconductive adhesive and a ladder filter using it. More specifically, it relates, for example, to an anisotropically electroconductive adhesive which is electroconductive in the direction of thermocompression bonding but is insulative in the other directions, and to a ladder filter using it.
2. Description of the Related Art
It is known that an electroconductive adhesive is used for conventionally bonding an external electrode formed on an piezoelectric resonator to a land formed on an insulating substrate in a ladder filter, that is, a piezoelectric part in which plural piezoelectric resonators utilizing mechanical resonance of a piezoelectric material are used (cf. Japanese Unexamined Patent Application Publication No. 11-168348).
The structure of this ladder filter will be explained. FIG. 1 is a top plan view showing an example of a ladder filter as disclosed in Japanese Unexamined Patent Application Publication No. 11-168348. FIG. 2 is its front view. A known insulating substrate such as a glass epoxy substrate or an alumina substrate is used as the insulating substrate b.
On one of the main surfaces of the insulating substrate b, four pattern electrodes c1, c2, c3 and c4 are formed with specific distances therebetween. On these pattern electrodes c1 through to c4, five lands e1, e2, e3, e4 and e5 are formed in line and with specific distances therebetween, wherein each of the lands e1 through to e4 is formed on one end of each of the pattern electrodes c1 through to c4, and the land e5 is formed on the other end of the pattern electrode c2.
On the lands e1 through to e5 of these pattern electrodes c1 through to c4, four piezoelectric resonators a1, b1, b2 and a2 are aligned in line and in this order, wherein the two piezoelectric resonators a1 and a2 are used as serial resonators having the same structure, and the other two piezoelectric resonators b1 and b2 are used as parallel resonators having the same structure as shown in FIG. 3.
Furthermore, in this ladder filter, the central portion in the lengthwise direction (in the up/down direction on the surface of FIG. 1) of an external electrode f1 of the piezoelectric resonator a1 which is to be the first serial resonator, is bonded to the land e1 of the pattern electrode c1 with an electroconductive adhesive d. Thereby, the external electrode f1 of the piezoelectric resonator a1 is connected to the pattern electrode c1.
Also, the central portion in the lengthwise direction of an external electrode f2 of the piezoelectric resonator a1, and the central portion in the lengthwise direction of an external electrode f1 of the piezoelectric resonator b1 which is to be the first parallel resonator, are bonded to the land e2 of the pattern electrode c2 with the electroconductive adhesive d. Thereby, the external electrode f2 of the piezoelectric resonator a1 and the external electrode f1 of the piezoelectric resonator b1 are connected to the pattern electrode c2.
Furthermore, the central portion in the lengthwise direction of an external electrode f2 of the piezoelectric resonator b1, and the central portion in the lengthwise direction of an external electrode f1 of the piezoelectric resonator b2 which is to be the second parallel resonator, are bonded to the land e3 of the pattern electrode c3 with the electroconductive adhesive d. Thereby, the external electrode f2 of the piezoelectric resonator b1 and the external electrode f1 of the piezoelectric resonator b2 are connected to the pattern electrode c3.
Also, the central portion in the lengthwise direction of an external electrode f2 of the piezoelectric resonator b2, and the central portion in the lengthwise direction of an external electrode f1 of the piezoelectric resonator a2 which is to be the second serial resonator, are bonded to the land e4 of the pattern electrode c4 with the electroconductive adhesive d. Thereby, the external electrode f2 of the piezoelectric resonator b2 and the external electrode f1 of the piezoelectric resonator a2 are connected to the pattern electrode c4.
Furthermore, the central portion in the lengthwise direction of an external electrode f2 of the piezoelectric resonator a2 is bonded to the land e5 of the pattern electrode c2 with the electroconductive adhesive d. Thereby, the external electrode f2 of the piezoelectric resonator a2 is connected to the pattern electrode c2.
However, as miniaturization of products has accompanied a shortened distance between the lands formed on an insulating substrate, there is danger, with the conventional electroconductive adhesive, that an electroconductive adhesive spills out or exudes out of the adhesion area on heating and pressing, easily entailing short circuitry between the lands. Thereupon, in the course of the investigation related to the present invention, bonding with an anisotropically electroconductive adhesive was investigated for preventing this short circuitry between the lands.
The anisotropically electroconductive adhesive for use herein is a thermosetting insulating adhesive with which electroconductive particles are blended. Accordingly, by interposing an anisotropically electroconductive adhesive between an electronic part such as a circuit board and another electronic part such as a circuit board for thermocompression bonding, the electroconductive particles are contacted with one another in the direction of thermocompression to electrically connect both electronic parts, while both electronic parts are connected mechanically with the thermosetting insulating adhesive. On the other hand, the electroconductive particles are not contacted with one another in the directions other than the direction of compression bonding, resulting in an insulated state.
When such a thermosetting, anisotropically electroconductive adhesive interposes and is thermocompressed between both electronic parts, the curing agent causes a curing reaction to cure the thermosetting resin and to bond both electronic parts. In many cases, an epoxy type thermosetting resin is used for the thermosetting insulating adhesive. To be more specific, an epoxy resin in combination with a curing agent for an epoxy resin selected from the group of various materials consisting of a polyamide resin, an amine resin, an imidazole resin, a melamine resin, an anhydride, etc., is used.
However when a conventional anisotropically electroconductive adhesive was used for bonding a piezoelectric resonator to a pattern electrode on an insulating substrate, a problem of degraded filter characteristics would occur in a ladder filter shown in FIG. 1. It was found in the course of the investigation related to the present invention, that it was caused by the vibration of the piezoelectric resonator propagated through the anisotropically electroconductive adhesive to the insulating substrate, which was then propagated to another piezoelectric resonator.
Thus, one of the main objects of this invention is to provide an anisotropically electroconductive adhesive which furnishes good filter characteristics without allowing the vibration of a piezoelectric resonator to be propagated to the insulating substrate, in the connection of the piezoelectric resonator with a pattern electrode on the insulating substrate.
Another object of the present invention is to provide a ladder filter having good filter characteristics, by using such an anisotropically electroconductive adhesive.
One aspect of the present invention is an anisotropically electroconductive adhesive comprising a thermosetting insulating adhesive mixed with electroconductive particles, the thermosetting insulating adhesive having an intrinsic acoustic impedance of about 1.4 MPaxc2x7s/m or less.
It is preferable that the thermosetting insulating adhesive in such an anisotropically electroconductive adhesive have a rubber elasticity.
A silicone rubber, a urethane rubber, an isobutylene rubber, or a mixture of two or more of them can be used as the thermosetting insulating adhesive.
Furthermore, resin particles covered with a metal are preferably used as the electroconductive particles.
An anisotropically electroconductive adhesive comprising such metal-covered resin particles preferably further comprises a filler made of inorganic particles having a particle size smaller than that of the resin particles.
Furthermore, another aspect of the present invention is a ladder filter comprising an insulating substrate on which pattern electrodes are formed, and plural piezoelectric resonators connected serially or in parallel to form a ladder type circuit on the insulating substrate, wherein the piezoelectric resonators and the pattern electrodes are electroconductively bonded with each other with the above-described anisotropically electroconductive adhesive.
Regarding the anisotropically electroconductive adhesive according to the present invention, as a result of investigations made on the relationships between the physical properties of a thermosetting insulating adhesive in an anisotropically electroconductive adhesive, and the propagation of the vibration of piezoelectric resonators or the filter characteristics of a ladder filter, it was found that the less the intrinsic acoustic impedance of the thermosetting insulating adhesive was matched with the intrinsic acoustic impedance of an element, the more enhanced the effect for preventing from the propagation of the vibration was and the better the filter characteristics. Thus the present invention has been accomplished.
It was found that for use in manufacturing a filter using piezoelectric resonators of this type, an anisotropically electroconductive adhesive comprising electroconductive particles and a thermosetting insulating adhesive, the thermosetting insulating adhesive having an intrinsic acoustic impedance of about 1.4 MPaxc2x7s/m or less, is preferable as the anisotropically electroconductive adhesive which can prevent from propagation of the vibration.
To be specific, a silicone rubber, a urethane rubber and an isobutylene rubber are examples of the resin used for the thermosetting insulating adhesive which has an intrinsic acoustic impedance xcfx81c (wherein xcfx81 representing specific gravity and c representing the speed of sound) which is greatly mismatched with that of a piezoelectric resonator using lead titanate zirconate.
By using, as the electroconductive particles, resin particles covered with a metal, electroconductivity is realized at the time of thermocompression bonding when the resin particles contact with one another to deform their shape. Furthermore, even when the thermosetting insulating adhesive expands thermally, the electroconductivity is maintained until the deformation of the resin particles has disappeared and the original shapes have been returned to result in loss of the contact between the resin particles. Thus, accurate electroconductivity can be obtained in spite of temperature fluctuation.
Furthermore, a filler is preferably compounded into this anisotropically electroconductive adhesive for preventing from exudation of the adhesive and precipitation of the electroconductive particles. Insulating inorganic particles can be used as the filler. As the examples, silicon dioxide, calcium carbonate, aluminum oxide, etc. are enumerated. It is necessary that these insulating inorganic particles should have a particle size smaller than that of the electroconductive particles. It is preferable that they have an average particle size in the range of from about 0.01 to 3 xcexcm.